This invention relates in general to self-balancing impedances bridges, and in particular to such bridges adapted for temperature control at extremely low temperatures, e.g. at milliKelvin levels.
FIG. 1 herein illustrates a prior art AC (alternating current) impedance bridge useful for monitoring the temperature of a cryogenic environment by measuring the resistance of a temperature dependent resistor RX in the environment. At low temperatures, below 1 degree kelvin, the prior art cannot measure high value resistors of above 100 K (kilo) ohms with sufficiently low power while achieving acceptable high resolution. The prior art works effectively at low resistance of below 10 K ohms, but modem resistive temperature sensors have values in the 100 K ohms to a few megohms, and many cryogenic applications and experiments need extremely low excitation powers to excite the resistive sensors in order to avoid self-heating of the sensors. The need for low excitation powers for the higher ohmage sensors has raised the problem of excess DC (direct current) bias currents from the prior arts"" preamplifiers 6. Sensors need to be measured with extremely low input voltage noise, on the order of below 10 nanovolts (RMS, root mean square, per square root Hertz at typically 16 Hz) along with low current noise of below 10 femto amps. At low temperatures in the millikelvin range, these sensors produce thermal noise, for example, approximately 1 to 4 nanovolts per square root Hz (Hertz) at 300 millikelvin for a 100 K ohm to 1 M (megohm) ohm resistor. The prior art is lacking in that the DC bias current is sufficiently high to self-heat the higher ohmage sensors. For accurate temperature measurements in the millikelvin range, the DC bias current flowing through a sensor needs to be near 1 pico amp or less. The prior art has bias currents in excess of 1 pico amp.
One solution would be to use a semiconductor operational amplifier with a bias current of the required 1 pico amp or less as the preamplifier 6, but unfortunately such amplifiers have an unacceptably high input noise voltage. This is because there is an inherent characteristic in such amplifiers: the lower the DC bias current, the higher the input voltage noise.
This invention overcomes these problems, and others, by providing a circuit that eliminates excess DC bias current while avoiding any significant increase in noise voltage communicated to a preamplifier.
An object of this invention is to provide a means and method for eliminating undesirable DC bias current from the input of a preamplifier according to this invention without an undesirable increase in measurement noise.
A further object of this invention is to provide a means and method for eliminating undesirable DC current from the input of a preamplifier according to this invention without undesirable loading the unknown impedance.
These objects, and others expressed or implied in this document, are accomplished by a bridge including an amplifier (commonly called a preamplifier) having gain and including differential inputs; a transformer, a secondary of the transformer being connected in series with the unknown impedance, the terminals of the series connection communicating with respective differential inputs of the amplifier; a transfer function, communicating with the output of the amplifier and exciting the primary of the transformer, for automatically driving the bridge toward balance, balance being achieved when the signal across the secondary equals the signal across the unknown impedance; and a circuit for blocking any DC bias current (direct current caused by an inherent bias of the inputs of the amplifier) from flowing through the unknown impedance. In a preferred embodiment the blocking circuit includes a pair of direct current blocking capacitors interposed between respective terminals of the series connection and the amplifier inputs, and a current shunt communicating with each capacitor on the capacitor""s amplifier side. Also in the preferred embodiment an equalizing transfer function includes a voltage follower whose input communicates with the unknown impedance""s terminal of the series connection, and whose output communicates with said each capacitor through said each capacitor""s current shunt, the input impedance of the follower being high enough and bias current being low enough to avoid significantly loading and heating the unknown impedance. In alternative embodiments, one end of the unknown impedance communicates with a reference potential, e.g. analog ground, so no DC bias current can be sourced from that end, and DC bias current is blocked from the other end by a blocking circuit which is equalized in signal potential by the equalizing transfer function.
This invention also presents a preamplifier configured from two sets of paralleled operational amplifiers feeding respective amplifiers which in turn feed the differential inputs of a single amplifier to convert the preamplifier output to single-ended. The paralleled input amplifiers substantially reduce voltage noise over conventional amplifiers, while the concurrent increase in DC bias current is not detrimental to a bridge according to this invention due the bias current elimination feature of this invention.